Build Instructions
------------------

* Requirements
  ------------

  - A Linux-based system with a standard C development environment
  - GNU make version 3.81 or later
  - skalibs version 2.8.0.0 or later: https://skarnet.org/software/skalibs/
  - execline version 2.5.1.0 or later: https://skarnet.org/software/execline/
  - s6 version 2.8.0.0 or later: https://skarnet.org/software/s6/
  - Optional: nsss version 0.0.1.1 or later: https://skarnet.org/software/nsss/
  - Optional: utmps version 0.0.2.0 or later: https://skarnet.org/software/utmps/

 This software is Linux-specific. It will run on a Linux kernel,
version 2.6.32 or later. However, it should not be too hard to port to
other Unix-like operating systems.


* Standard usage
  --------------

  ./configure && make && sudo make install

 will work for most users.
 It will install the exported binaries in /bin, the unexported
binaries in /libexec, and the skeleton scripts in /etc/s6-linux-init/skel.

 You can strip the binaries and libraries of their extra symbols via
"make strip" before the "make install" phase. It will shave a few bytes
off them.

 Your default init binaries will not be overwritten. They will only be
overwritten by a manual action after you invoke s6-linux-init-maker,
check that you are happy with its output, and decide to use it as
your default init.


* Customization
  -------------

 You can customize paths via flags given to configure.
 See ./configure --help for a list of all available configure options.


* Environment variables
  ---------------------

 Controlling a build process via environment variables is a big and
dangerous hammer. You should try and pass flags to configure instead;
nevertheless, a few standard environment variables are recognized.

 If the CC environment variable is set, its value will override compiler
detection by configure. The --host=HOST option will still add a HOST-
prefix to the value of CC.

 The values of CFLAGS, CPPFLAGS and LDFLAGS will be appended to flags
auto-detected by configure. To entirely override the flags set by
configure instead, use make variables.


* Make variables
  --------------

 You can invoke make with a few variables for more configuration.

 CC, CFLAGS, CPPFLAGS, LDFLAGS, LDLIBS, AR, RANLIB, STRIP, INSTALL and
CROSS_COMPILE can all be overridden on the make command line. This is
an even bigger hammer than running ./configure with environment
variables, so it is advised to only do this when it is the only way of
obtaining the behaviour you want.

 DESTDIR can be given on the "make install" command line in order to
install to a staging directory.


* Shared libraries
  ----------------

 Software from skarnet.org is small enough that shared libraries are
generally not worth using. Static linking is simpler and incurs less
runtime overhead and less points of failure: so by default, shared
libraries are not built and binaries are linked against the static
versions of the skarnet.org libraries. Nevertheless, you can:
  * build shared libraries: --enable-shared
  * link binaries against shared libraries: --disable-allstatic


* Static binaries
  ---------------

 By default, binaries are linked against static versions of all the
libraries they depend on, except for the libc. You can enforce
linking against the static libc with --enable-static-libc.

 (If you are using a GNU/Linux system, be aware that the GNU libc
behaves badly with static linking and produces huge executables,
which is why it is not the default. Other libcs are better suited
to static linking, for instance musl: http://musl-libc.org/)


* Cross-compilation
  -----------------

 skarnet.org packages centralize all the difficulty of
cross-compilation in one place: skalibs. Once you have
cross-compiled skalibs, the rest is easy.

 * Use the --host=HOST option to configure, HOST being the triplet
for your target.
 * Make sure your cross-toolchain binaries (i.e. prefixed with HOST-)
are accessible via your PATH environment variable.
 * Make sure to use the correct version of skalibs for your target,
and the correct sysdeps directory, making use of the
--with-include, --with-lib, --with-dynlib and --with-sysdeps
options as necessary.


* The slashpackage convention
  ---------------------------

 The slashpackage convention (http://cr.yp.to/slashpackage.html)
is a package installation scheme that provides a few guarantees
over other conventions such as the FHS, for instance fixed
absolute pathnames. skarnet.org packages support it: use the
--enable-slashpackage option to configure, or
--enable-slashpackage=DIR for a prefixed DIR/package tree.
This option will activate slashpackage support during the build
and set slashpackage-compatible installation directories.
If $package_home is the home of the package, defined as
DIR/package/$category/$package-$version with the variables
read from the package/info file, then:

  --dynlibdir is set to $package_home/library.so
  --bindir is set to $package_home/command
  --sbindir is also set to $package_home/command (slashpackage
differentiates root-only binaries by their Unix rights, not their
location in the filesystem)
  --libexecdir is also set to $package_home/command (slashpackage
does not need a specific directory for internal binaries)
  --libdir is set to $package_home/library
  --includedir is set to $package_home/include

 --prefix is pretty much ignored when you use --enable-slashpackage.
You should probably not use both --enable-slashpackage and --prefix.

 When using slashpackage, two additional Makefile targets are
available after "make install":
 - "make update" changes the default version of the software to the
freshly installed one. (This is useful when you have several installed
versions of the same software, which slashpackage supports.)
 - "make -L global-links" adds links from /command and /library.so to the
default version of the binaries and shared libraries. The "-L" option to
make is necessary because targets are symbolic links, and the default make
behaviour is to check the pointed file's timestamp and not the symlink's
timestamp.


* Absolute pathnames
  ------------------

 You may want to use fixed absolute pathnames even if you're not
following the slashpackage convention: for instance, the Nix packaging
system prefers calling binaries with immutable paths rather than rely on
PATH resolution. If you are in that case, use the --enable-absolute-paths
option to configure. This will ensure that programs calling binaries from
this package will call them with their full installation path (in bindir)
without relying on a PATH search.


* Out-of-tree builds
  ------------------

 skarnet.org packages do not support out-of-tree builds. They
are small, so it does not cost much to duplicate the entire
source tree if parallel builds are needed.
